1. Field of the Invention
The present invention relates to a steering mechanism for steerable trailers which are used for transporting long or heavy loads. More particularly, the present invention is directed to a steering mechanism which generates an angle of inclination in steerable wheels of each fixed steerable axle mounted to the steerable trailer. The angle of inclination generated in the fixed steerable axle is in proportion to the relative angle between a towing vehicle and the steerable trailer during the negotiation of a turn.
2. Description of the Prior Art
A conventional towing vehicle used to tow a loadbearing trailer generally includes a "fifth-wheel" mounted to the towing vehicle generally above a set of driving axles of the towing vehicle. The load-bearing trailer is attached to the towing vehicle by mounting a conventional trailer kingpin and a locking dog of the trailer into an approach slot of the towing vehicle fifth wheel. The towing vehicle fifth-wheel allows relative pivotal motion between the towing vehicle and the trailer and the fifth-wheel also provides a load bearing surface on which a forward end of the load-bearing trailer is supported. Load-bearing trailers usually include at least one fixed, non-steerable axle with ground engaging wheels. The fixed, non-steerable axle is mounted to the load-bearing trailer by conventional suspension means. The fixed, non-steerable axle, together with the towing vehicle fifth wheel plate, support the loadbearing trailer. The weight and length of a load carried by the load-bearing trailer generally determines both the required length of the trailer and the number and location of the fixed, non-steerable axle mounted to the trailer.
For a conventional load which is typically light or relatively short, the fixed non-steerable axle can be mounted on the trailer close enough to the fifth wheel of the towing vehicle that the trailer can be safely negotiated through relatively sharp turns by a towing vehicle without encountering problems experienced by trailers used to carry longer or heavier loads. Such problems experienced by a longer trailer, for example, include the tendency of the trailer to roll over inside corners during the negotiation of the trailer through a turn. In addition, other problems arise, as explained below, whenever such trailers are designed for transporting heavy loads, such as military tanks.
For either long or heavy loads, a load bearing trailer requires a greater number of axles to support the trailer than are required for trailers used to haul conventional loads. Thus, as the number of axles increases, both the length and load weight capacity of the trailer may be increased. However, increasing the number of axles on a trailer will also create other problems. If a trailer includes numerous non-steerable axles, the longitudinal axes of which are all aligned in perpendicular relationship to a longitudinal axis of the trailer, the trailer cannot be negotiated through a turn without imposing varying degrees of undesirable stresses on the axles. During the turn, some or all axles may be forced to move or skid in directions including those which are other than parallel to the longitudinal axis of the trailer, thus generating the undesirable stresses. The stresses are undesirable because conventional axles, wheels, and trailer suspension systems supporting such axles are usually designed for only a nominal amount of such skidding stress. Problems usually arise which include excessive wear to the ground engaging tires forced to skid during a turn. Other problems arise because non-steerable axles have a tendency to travel straight forward through a curve and negotiating such a trailer through a curve, especially at higher speeds, may cause severe safety hazards. Yet another problem arising is that the energy demands on a towing vehicle used to tow such a trailer through turns are greater because of energy losses associated with tire skidding as the trailer negotiates a turn. To accommodate higher energy demands, the towing vehicle must have a strong structural design, thus increasing the cost of the towing vehicle.
Steerable trailers have been proposed in the prior art to overcome the problems referenced above. Steerable trailers generally include steerable and non-steerable axles mounted to the trailer. The steerable axles are controlled by various steering mechanisms, a steering output signal from which is proportional to a relative angle between the towing vehicle and the trailer. The steering output signals from the steering mechanisms variously include mechanical, electrical, and hydraulic output signals. The steering mechanism output signals are received by the steerable axles, generating an angle of inclination therein. The angle of inclination for a given output signal will generally vary according to the geometric configuration of the steerable axle, the location of the axle on the steerable trailer, and the relative angle between the towing vehicle and the steerable trailer.
Generally, steerable trailers proposed by the prior art are for use with a trailer design having a frame of a sufficient height to allow full pivotal rotation of all steerable axles ("pivotal steerable axle"), about the axle's mid-point and beneath the trailer. However, such pivotal steerable axles cannot always be used with certain low-to-the-ground trailers. A low-to-the-ground trailer design are often necessary to allow for bridge clearance by loads having considerable height. In addition, pivotal steerable axles structurally adequate to withstand the extreme force imposed on trailer axles during transportation of heavy loads are very costly. A conventional alternative, under these circumstances, is steerable axles which incorporate a fixed, steerable axle design with steerable wheels which may be inclined at outboard ends of the fixed axle. An angle of inclination for the wheels of such a fixed axle design is about a wheel kingpin assembly mounted at each outboard end of the fixed steerable axle. For such a fixed axle design, the axle can be of a sufficient length so that the low-to-the-ground trailer frame does not interfere with the inclination of the wheels about the wheel kingpins.
Problems arise, however, when prior art steering mechanisms are used to steer such fixed, steerable axles because of inherent physical limitations on the extent of inclination which can be achieved in such wheel kingpin assemblies associated with the fixed, steerable axle designs. Prior art steering mechanisms cannot generally be used with fixed steerable axles because, as will be more fully explained below, prior art mechanisms develop a steering output signal for all relative angles developed between the towing vehicle and the steerable trailer during a turn. Conventional fixed, steerable axles are designed so that the angle of inclination generated in the steerable axle by an output signal from a prior art steering mechanism, for a given turning radius, increases as the distance between steerable axle and the fixed non-steerable axle increases. Thus, as a turning radius approaches 90 degrees, the angle of inclination generated in a fixed, steerable axle, sufficiently distant from the fixed, nonsteerable axle, will also approach 90 degrees. However, because of the inherent physical limitations referenced above, the wheels mounted on wheel kingpins of a fixed, steerable axle cannot be inclined to an angle of 90 degrees. While the actual inherent limit on the angle of inclination will vary somewhat, depending on the specific fixed steerable axle design, the limit of inclination will generally be about 45 degrees. Further, although a fixed steerable axle can be designed to receive any output signal from a steering mechanism and never exceed a maximum angle of inclination, such as 45 degrees, to do so would generally be at the expense of steering precision, especially in the lower ranges of turns. Steering precision is most important in the lower ranges principally because the radius of most turns negotiated by a towing vehicle is less than 45 degrees.
Finally, many prior art steering mechanisms generate an output signal which fails to impose an actual angle of incidence in a steerable axle which sufficiently approximates a theoretical angle of incidence in a desired range of relative angles between the towing vehicle and the steerable trailer. The extent of error between the actual angle of incidence generated in the steerable axle and the theoretical angle of incidence in the steerable axle will vary according to the type of steering mechanism chosen, to control the steerable trailer.
The problems arising from the inherent physical limitation of a fixed, steerable axle are more fully illustrated below by reference to a comparison between the theoretical angles of inclination, and actual angles of inclination generated in both pivot steerable axles and fixed steerable axles by prior art steering mechanisms. A comparison of the extent of the error between the actual angle of inclination and the theoretical angle of inclination for a given steerable axle is also presented below for two generalized forms of prior art steering mechanisms.
The theoretical angle of incidence for a particular steering axle is a function of the relative angle between the towing vehicle and the trailer and is also a function of the steering axle geometry and the position of the axle on the trailer. FIG. 1 shows a top plan view of a conventional towing vehicle/steerable trailer assembly. The concept of a theoretical angle of incidence is explained below by reference to a conventional towing vehicle steerable trailer assembly as shown in FIG. 1.
An assembly 10 comprises a towing vehicle 12 and a steerable trailer 13. The towing vehicle 12 comprises front steerable wheels 14, a body 15 and a rear driving axle 16 with rear axle wheels 17. A fifth-wheel 18 is positioned over the rear axle 16 with its central axis positioned at the mid-point of the rear axle 16. The trailer 13 comprises a frame 20 with a fixed non-steerable axle 21, a steerable axle 22 and, a kingpin 23. A pivot point C.sub.2, of the steerable axle 22 is a distance, d.sub.1, from the fixed nonsteerable axle 21. The fixed non-steerable axle 21 is a distance, d.sub.2, from the trailer kingpin 23. Distance, d.sub.3, is the lateral distance between the longitudinal axis, b, of trailer 13 and the pivot center C.sub.2 of a particular wheel assembly 24 of a steerable axle 22. For a pivot axle such as steerable axle 22, d.sub.3 =0. Wheel assemblies 24 are rotatably attached by conventional means to each end of each axle. Wheel assemblies 24 comprise conventional dual wheels but it is foreseen that the wheel assembly 24 may comprise a single wheel.
When a towing vehicle 12 towing a steerable trailer 13 negotiates a turn, it does so about an instantaneous turning center, C.sub.1, that is a point about which all wheels of the towing vehicle 12 revolve in negotiating a turn of constant radius. If each wheel assembly 24 of the steerable axle 22 of trailer 13 is properly inclined for a turn of constant radius established by the towing vehicle 12, the turning center of each circular path traveled by each wheel assembly 24 will also be at C.sub.1. If a center axis of a circular path traveled by a wheel assembly 24 does not focus on the turning center, C.sub.1, the wheel assembly 24 will be misaligned for the particular turning radius. During the turn, the misaligned wheel assemblies 24 will thus undergo skidding and scuffing and the associated axle will be exposed to undue stresses. There is a theoretical angle of incidence, Phi.sub.(theo), for each wheel assembly 24 of each steerable axle 22 which, if adopted by the wheel assembly 24, will align the center axis of the path traveled by the wheel assembly 24 on turning center, C.sub.1, and will avoid the imposition of scuffing and of undue stresses.
The theoretical angle, Phi.sub.(theo), of a steerable axle 22 located between the kingpin 23 and the fixed axle 21 can be calculated and is a function of distances d.sub.1, d.sub.2, d.sub.3 and a relative angle, Theta, between the towing vehicle 12 and the trailer 13. To calculate Phi.sub.(theo), the following assumptions are made: (1) the longitudinal axis, a, of the fixed axle 21 is at right angles to the longitudinal axis, b, of the trailer 13; and (2) when the towing vehicle 12 has established a constant turning radius, the longitudinal axis, a, of the fixed axle 21 will, if extended, intersect C.sub.1.
For such conditions, Phi.sub.(Theo) is mathematically expressed as follows: ##EQU1## where Phi.sub.(theo) =theoretical angle for a given wheel assembly 24 of a steerable axle 22
d.sub.1 =distance between fixed axle 21 and a steerable axle 22 pivot point, C.sub.2 PA0 d.sub.2 =distance between trailer 13, kingpin 23 and fixed axle 21 PA0 d.sub.3 =lateral distance between the longitudinal axis, b, of trailer 13 and the pivot center, C.sub.2, of a particular wheel assembly 24 of a steerable axle 22 (d.sub.3 =0 in this example). PA0 Theta=angle between towing vehicle 12 and trailer 13 PA0 1.sub.1 =one-half of longitudinal length of turning arm 27 PA0 1.sub.2 =one-half of longitudinal length of turning arm 29 PA0 r.sub.4 =radius of forward turning gear 41 PA0 r.sub.5 =radius of rear turning of gear 42 PA0 Theta=angle between towing vehicle 12 and trailer 13 in radians.
Note that d.sub.1, d.sub.2, and d.sub.3 are mathematical constants for each wheel assembly 24 of each steerable axle 22.
A plot of the angle Phi(theo) for the steerable axle corresponding to each angle Theta between 0 and 90 degrees is presented in FIG. 2. The values corresponding to the plot in FIG. 2 are presented below in Table 1.
TABLE 1 ______________________________________ Phi.sub.(theo) for given Theta (d.sub.1 = 15, d.sub.2 - 26.5, d.sub.3 = 0) (Angles are in Degrees) THETA PHI.sub.(theo) ______________________________________ 0 0 5 3 10 6 15 9 20 12 25 15 30 18 35 22 40 25 45 30 50 34 55 39 60 44 65 51 70 57 75 65 80 73 85 81 90 90 ______________________________________
Note that when Theta is 0 degrees and 90 degrees, Phi.sub.(theo) is also 0 degrees and 90 degrees, respectively. However, in the range of Theta between 0 to 90 degrees, Phi.sub.(theo) is non-linear.
Various turning mechanisms have been proposed for transforming the relative angle between the towing vehicle 12 and trailer 13, into a steering mechanism output signal which inclines the wheel assemblies 24 of the steerable axles 22, which are pivot axles. The two most prevelant prior art steering mechanisms are discussed and, for convenience, are classified in two categories, herein named after the mathematical relationship describing their behavior.
Many prior art steerable trailers incorporate the pivot axle with a steering mechanism, the output from which is related to the sine of the angle Theta between the towing vehicle and the steerable trailer. Such steering mechanisms are hereinafter referred to as "sine steering mechanisms". FIG. 3 shows a top plan view of the conventional towing vehicle/steerable trailer assembly of FIG. 1 with a "sine steering mechanism" used in conjunction with a pivot axle. Referring to FIG. 3, an example of a sine turning mechanism 25 for a pivot axle is one which comprises a form of a forward turning arm 27, a pair of motion transfer rods 28, and a rear turning arm 29, all pivotally attached in a general form of a parallelogram.
The forward turning arm 27 is pivotally attached to the trailer frame 20 at a forwardmost position of frame 20 with the forward turning arm 27 centered in spaced relation above the kingpin 23. The kingpin 23 extends downwardly into the pivot point of the fifth wheel 18. The forward turning arm 27 is releasably attached to the fifth-wheel 18 in such a way as to maintain a longitudinal axis, e, of the forward turning arms 27 in vertical alignment with the longitudinal axis, f, of the rear axle 16 of the towing vehicle 12 as the towing vehicle 12 moves into angular relationship with the trailer 13 during the negotiation of a turn.
As an angle Theta is developed between the towing vehicle 12 and the trailer 13, a steering mechanism 25 output signal is generated as the forward turning arm 27 is rotated by the towing vehicle 12, relative to the trailer 13. The rotational motion of the forward turning arm 27 is transmitted to linear motion in the motion transfer rods 28. The linear motion of the transfer rods 28 is transmitted to rotational motion in the rear turning arm 29 which is pivotally attached to the trailer 13 and fixedly attached to the steerable axle 22.
The relationship between an angle Phi(sin) in the steerable axle 22 imposed by the sine steering mechanism 25, and Theta is expressed below: ##EQU2##
Where:
FIG. 4 shows a plot of the values of Phi.sub.(sin) and Phi.sub.(theo) for steerable axle 22 corresponding to each value of Theta between 0 and 90 degrees.
The value of Phi.sub.(sin), Phi.sub.(theo), and Theta corresponding to the plot of FIG. 4 are presented in Table 2.
TABLE 2 ______________________________________ SINE STEERING MECHANISM (d.sub.1 = 15, d.sub.2 = 26.5, d.sub.3 = 0, 1.sub.1 = 12, 1.sub.2 = 17.25) THETA PHI.sub.(theo) PHI.sub.(sin) ERROR PHI ______________________________________ 0 0 0 0 5 3 3 -1 10 6 7 -1 15 9 10 -2 20 12 14 -2 25 15 17 -2 30 18 20 -2 35 22 24 -2 40 25 27 -1 45 30 29 0 50 34 32 2 55 39 35 4 60 44 37 7 65 51 39 11 70 57 41 16 75 65 42 22 80 73 43 29 85 81 44 37 90 90 44 46 ______________________________________
The values of 1.sub.1 and 1.sub.2 were chosen in this example such that: Phi.sub.(sin) =Phi.sub.(theo) at Theta=45 degrees. For such values of 1.sub.1 and 1.sub.2, the sine steering mechanism 25 generates an error which is generally constant for Theta between 0 and 45 degrees. However, at angles of Theta beyond 45 degrees the error begins to grow and achieves a maximum at Theta of 90 degrees.
As previously discussed, a problem arises in using a sine steering mechanism 45 with a pivot axle when the particular load requires that a trailer load carrying surface be low to the ground. For such trailer designs, a trailer frame may prohibit inclination of the pivot steerable 22 axle beneath the frame. Thus, for such low-to-the-ground trailer designs, pivot axles generally cannot be used with a conventional trailer frame 20, such as that disclosed in FIG. 3. Further, inclination in any steerable wheels can generally occur only at the outboard ends of a steerable axle thus requiring some form of fixed-steerable axle. Wheels for such fixed-steerable axles are mounted on kingpins at each end of the axle. But the use of wheel kingpin designs will impose a limit on the extent to which the steerable wheel may be inclined. Conventional kingpin wheel designs cannot be inclined beyond about 45 degrees. As such, sine steering mechanisms cannot generally be used with steerable trailers having multiple fixed steerable axles located at various distances from the fixed nonsteerable axle because, as Theta approaches 90 degrees, Phi.sub.(sin) may exceed 45 degrees for a fixed-steerable axle positioned sufficiently distant from the fixed non-steerable axle.
The Felborn '428 patent discloses a trailer steering mechanism which is an example of a sine steering mechanism. The steering mechanism comprises a plate member 80 fixedly mounted to a kingpin 66 which is caused to rotate with the towing vehicle's fifth wheel. Steering rods 140 and 142 are pivotally attached at a first end to the rear margin of the plate 80. The second ends of the rods 140 and 142 are pivotally attached to an axle 46 which is a steerable pivot axle which can be made to rotate about a pivot point corresponding to kingpin 42. Further, the axle 46 is in spaced relation beneath the frame member 10 of the trailer to allow sufficient clearance between steerable wheels 16 and the trailer frame members 10 when the steerable axle 46 is rotated. As can be noted from FIG. 2 of the '428 Felborn patent, not only does such a sine mechanism require clearance beneath a trailer for the rotation of the steerable pivot axle, sufficient clearance must be provided also for lateral movement of the steering rods 142 and 143 beneath a trailer.
Another steering system proposed in the prior art for steerable trailers is a "radian steering mechanism" for pivot axles. FIG. 5 shows an example of a radian steering mechanism 40 in place of the sine turning mechanism 25 in the conventional towing vehicle/steerable trailer shown in FIG. 3. As shown in FIG. 5, a form of a radian turning system 40 comprises a forward turning gear 41 with a radius r.sub.4, a rear turning gear 42 with a radius r.sub.5 and a chain 43. The forward turning gear 41 is pivotally mounted to the forward end of the towing vehicle 12 with a central axis aligning with the longitudinal axis of the kingpin 23. An output signal is generated in the radian steering mechanism 25 when rotational movement of the towing vehicle 12 in relationship to the steerable trailer 13 causes rotational movement of the forward turning gear 41. The central axis of the rear turning gear 42 is positioned centrally and in spaced relation above the steerable axle 22. The chain 43 of endless configuration connects the forward turning gear 41 to the rear turning gear 42. Rotational motion of the forward gear 41 is transmitted to linear motion in the chain 43. The linear motion in chain 43 causes rotational motion in the rear turning gear 42.
An angle Phi.sub.(rad) imposed by the radian steering mechanism 40 in the steerable axle 22 as a function of Theta is expressed below: ##EQU3##
Where:
FIG. 6 shows a plot of the Phi.sub.(rad) angle imposed by radian steering mechanism 40 for the angles of Theta in the range 0 to 90 degrees.
The values of Theta, Phi.sub.(theo), Phi.sub.(rad) which correspond to the plot of FIG. 6 are presented in Table 3.
TABLE 3 ______________________________________ RADIAN MECHANISM (d.sub.1 = 15, d.sub.2 = 26.5, d.sub.3 = 0, r.sub.4 = 12, r.sub.5 = 18) (ANGLES ARE IN DEGREES) THETA PHI.sub.(theo) PHI.sub.(rad) ERROR ______________________________________ 0 0 0 0 5 3 3 0 10 6 7 -1 15 9 10 -1 20 12 13 -2 25 15 17 -2 30 15 20 -2 35 22 23 -2 40 25 27 -1 45 30 30 0 50 34 33 1 55 39 37 2 60 44 40 4 65 51 43 7 70 57 47 11 75 65 50 15 80 73 53 19 85 81 57 25 90 90 60 30 ______________________________________
As with the sine steering mechanism 25, use of a radian steering mechanism 40 with a steerable axle 22 in the form of a pivot axle requires that the trailer frame 13 be of sufficient height to allow the rotation of the steerable axle 22 therebeneath. Thus, a radian steering mechanism cannot generally be used with a pivot axle on low-to-the ground trailers. Further, as previously discussed, a fixed steerable axle design cannot be used with a radian steering mechanism because such axles are physically limited to a range of inclination, typically, less than 45 degrees. A radian steering mechanism will attempt to generate angles of Phi.sub.(rad) greater than 45 degrees for angles of Theta between 0 and 90 degrees. Thus, the radian steering mechanism 40 cannot be used with a fixed steerable axle without a means to limit its output.
Further, radian steering mechanisms incorporating cables or chains suffer the disadvantages associated with stretching or wear, which result in worsening any imprecision of the steering mechanism. Finally, the radian steering mechanisms cannot be used without a major modification because the radian steering mechanism will be mounted to the gooseneck portions which is a different height than the steerable axle.
An example of a radian steering system with a pivot axle is disclosed in the Chung '596 patent. The '596 patent discloses a steering apparatus with a steering transmission plate 3 rotatably attached to the forward end of the trailer and a rear steering transmitting plate 11 rotatably attached to the trailer and located at the rear end of the trailer. The central axis of the forward plate 3 aligns with the central axis of the fifth wheel 1 and is keyed to the fifth wheel 1 when the trailer is attached to the towing vehicle. The plates are joined together by a cable arrangement wherein rotation of the front steering plate 3 causes a corresponding rotation in the rear steering plate 11. Note that because the cable 15 is crossed, the rotation of steering plate 11 will be in an opposite direction to that of steering plate 3. The pivot axle in the '596 patent comprises a pair of axles mounted to a rotary plate 11. The plate 11 is pivotally attached to the trailer.
Referring to FIGS. 4 and 6, note that the plot for Phi.sub.(sin) and Phi.sub.(rad) cross the plot for Phi.sub.(Theo) at points where Theta=45 degrees, respectively (hereinafter "crossover points"). The crossover point may be varied according to the design needs of the trailer. However, if the crossover point is established at a sufficiently low value of Theta to avoid Phi exceeding 45 degrees (as is necessary when fixed-steerable axles are used) when Theta nears 90 degrees the error between Phi.sub.(theo) and the angle Phi imposed by the steering mechanism may become excessive and unacceptable. If the crossover point is established at sufficiently high values of Theta to avoid excessive error between Phi.sub.(theo) and the angle Phi imposed by the steering mechanism for lower values of Theta, then at some value of Theta, the Phi imposed by the steering mechanism will exceed 45 degrees, a condition which is equally unacceptable in fixed steerable axles.
The U.S. Pat. No. 4,740,006 to Ducote discloses a device which is neither a sine nor a radian steering mechanism but is rather a steering mechanism which uses a microprocessor to generate a steering mechanism output signal. The devices disclosed in the '006 patent can theoretically be used to limit wheel angle of inclinations to 45 degrees and may also be used to approximate the theoretical angle Phi.sub.(theo). The '006 patent discloses a steering mechanism which incorporates a microprocessor, which based on an input from a sensor measuring the angle Theta, signals a servo mechanism to electrically drive a gear box which imposes an angle of inclination in the wheels of the fixed steerable axle. However, such electrical components are not reliable, especially in military applications where externally generated electromagnetic radiation may create undesirable electrical signals in electrical conductors, including those comprising the servo mechanisms, of the '006 steering mechanism, generating erroneous steering signals. Further, such servo mechanisms may not withstand other effects of torturous environments typically present in military applications. Finally, such designs require alternative sources of energy to incline the steerable axles. The alternative sources may be separate electrical generators to power the servo mechanisms. Also, such designs would require an electrical connection between the towing vehicle and the trailer making the design less reliable and also creates lag problems. If hydraulics are used to incline the steerable axles, then a separate source of hydraulic supply is required, which further reduces reliability and increases cost.
Therefore, the devices disclosed in the prior art as previously discussed, and also other devices which combine the effects of the sine steering mechanism and the radian steering mechanism cannot be used without suffering the disadvantage cited above.